dye-stimulated control of conducting polypyrrole morphology
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Dye-stimulated c
aDepartment of Physics and Measureme
University of Chemistry and Technology, 1
[email protected]; Fax: +420 220 444 334bFaculty of Mathematics and Physics, Cha
Republic. E-mail: [email protected] of Macromolecular Chemistry, Aca
162 06 Prague 6, Czech Republic. E-mail: tr
Cite this: RSC Adv., 2017, 7, 51495
Received 8th September 2017Accepted 17th October 2017
DOI: 10.1039/c7ra10027b
rsc.li/rsc-advances
This journal is © The Royal Society of C
ontrol of conducting polypyrrolemorphology
Stanislav Valtera,a Jan Prokes,b Jitka Kopecka,a Martin Vrnata,a Miroslava Trchova, c
Martin Varga,b Jaroslav Stejskal c and Dusan Kopecky *a
Azo dyes represent important structure-guiding agents which exhibit non-covalent interactions of various
types (ionic and hydrogen bonding, p–p stacking, hydrophobic interactions, etc.) allowing for their self-
assembly in aqueous solutions and the subsequent formation of seeds or templates for the preparation
of supramolecular structures of conducting polymers, especially polypyrrole (PPy). Three azo dyes (Acid
Red 1, Orange G and Sunset Yellow FCF) bearing hydrophilic functional groups, with mutually different
positions on a hydrophobic naphthylphenyldiazene skeleton, were used as structure-guiding agents in
the synthesis of highly organized supramolecular structures of PPy in aqueous media. The synthesized
polymers were studied by scanning electron microscopy, energy dispersive X-ray, and Fourier-transform
infrared and Raman spectroscopies. Measurement of the conductivity revealed a moderate value of
conductivity (around units of S cm�1) and reduced stability indicated by relatively fast conductivity decay.
Infrared spectroscopy indicated a lower doping level of all PPy prepared in the presence of tested dyes
compared to that of standard globular PPy. In contrast, Raman spectroscopy, which is a surface-sensitive
method, indicated a slightly higher protonation level compared to that of standard globular PPy or
nanotubular PPy synthesized in the presence of the well-known structure-guiding agent methyl orange.
This discrepancy in the obtained doping levels is discussed and some consequences between the doping
level of PPy and its conductivity are also pointed out.
1. Introduction
In recent years, the fabrication of nano- or microstructuredpolypyrrole (PPy) has become an issue of signicant researchinterest. Due to the specic conduction mechanism in PPy, itsunique electrical properties, the possibility to modulate theseelectrical properties by a reversible doping/dedoping process,1
and its controllable synthesis and processability, 1-D structuresof PPy in particular attract attention in the areas of nanoscienceand nanotechnology. There are numerous approaches whichprovide the possibility to control the morphology of the supra-molecular structure of synthesized polymers.2 To name justsome of them, hard-template methods (application of porousmembranes3), so-template methods (interfacial polymeriza-tion,4 dilute polymerization,5 ultrasonic shake-up,6 radiolyticsynthesis,7 and reverse emulsion polymerization8), and alsomodied electrochemical synthesis (electrospinning,9 and
nts, Faculty of Chemical Engineering,
66 28 Prague 6, Czech Republic. E-mail:
; Tel: +420 220 443 351
rles University, 180 00 Prague 8, Czech
demy of Sciences of the Czech Republic,
hemistry 2017
directed electrochemical nanowire assembly10) can bementioned.
The chemical synthesis of nano-structured PPy in the pres-ence of azo dyes represents a special category which falls intoso-template methods. Utilization of methyl orange (MO; AcidOrange 52) as a structure-guiding agent was rst reported by Huet al.11 and Yang et al.12 resulting in hollow nanotubes withcircular proles. In their pioneering work, Yang et al.12
proposed a mechanism which expected the polymerization totake place on the surface of self-assembledMO–iron(III) chloridecomplexes. Another representative of azo dyes, Acid Red 1 (AR),was applied a few years later to obtain rectangular nanotubes13
or nanobers.14 In the following years, several successfulattempts to synthesize 1-D PPy in the presence of MO andiron(III) chloride appeared.15–19 The “conventional” compositionof the reactionmixture was sometimes modied by the additionof a surfactant.20 Later, as reported by Kopecka et al.,21 thespectrum of proposed reactionmechanisms was complementedby the concept of a seeding mechanism. The current status ofthe synthesis of PPy nanotubes/nanowires using a reactionmixture containing pyrrole, MO and iron(III) chloride isreviewed in ref. 22. In this review, exclusive formation of 1-Dstructures (classied as either nanotubes or nanobers) is re-ported. A recent report by Sapurina et al.23 focused on tuning themorphology and conductivity by varying the temperature of
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synthesis (from subzero temperature, �24 �C, to 60 �C), and theselection of oxidants (iron(III) nitrate, sulfate, etc.) and dyes(indigo carmine, cresol red, thymol blue, etc.). Moreover, Liet al.24 have compared the synthesis of PPy in the presence ofMO and ethyl orange, and nally Kopecky et al.25 have founda power law describing the dependence of the conductivity onthe nanotube diameter for this class of materials.
To make some conclusions from the above-mentionedreports, the following aspects should be emphasized: (1) theinuence of the reaction parameters (concentrations andconcentration ratios of the reactants, sequence and manner oftheir addition, temperature of synthesis, total time of synthesis,stirring of native liquor, etc.) on polymer morphology. (2) Avariety of reaction mechanisms is proposed, accenting the roleof azo dye (formation of so templates by self-assembly of azodyes and subsequent polymerization on the surface of thesestructures, interaction of a pyrrole monomer with an azo groupvia H-bonding and subsequent polymerization of the pyrrole byreaction with a MO–Fe3+ associate, and polymerization insidethe AR micelles). It is rather surprising that the synthesis ofnanostructured PPy in the presence of azo dyes other than MOis rarely mentioned. An interesting result is presented in ref. 26,where Y-shaped or dendritic hollow structures of PPy weregrown on benzyl orange (BO) precipitates obtained in solutionscontaining BO–HCl or BO–FeCl3. The authors state that in therst step typical templates were formed and then hollowmicrometer-sized Y-junctions, with morphologies like thesetemplates, were produced. The second report concerning azodyes that are different from MO uses Remazol Black B (ReactiveBlack 5),27 but the authors prepared PPy by galvanostaticsynthesis, so their results are not relevant to our topic. It shouldbe noted that a dye closely related to MO, ethyl orange, does notsupport the formation of a 1-D morphology.24
With respect to the reaction mechanisms leading to the 1-DPPy structures that were proposed, some properties of azo dyesseem to be important: azo dyes (due to the presence of charac-teristic groups containing free electron pairs) easily formcomplexes with iron cations.28,29 Furthermore, there are rathercomplicated interactions between the azo dyes consisting ofplanar aromatic rings and substituted by polar groups which canbe hydrated by water molecules.30 In aqueous solutions, themajority of azo dyes associate by stacking mechanisms in a step-wise manner through dimers, trimers, etc., without evidence ofa critical micelle concentration (CMC). The stacking processes aresupported by acidication of the solution; the MO associates arestable at a pH lower than 4.3.24,31 It is well known that the poly-merization process takes place in an acidic environment and thatit is connected with releasing protons, leading to a furtherincrease in acidity. For some azo dyes, however, a balance betweenthe hydrophilic and hydrophobic part of their molecules playsa certain role and the behaviour of such azo dyes resemblessurfactants; they produce micelles and exhibit a CMC.32,33
Our previous papers21,25 were contributions to the discussionabout processes taking place during the synthesis of polypyrrolenanotubes in the presence of MO (or, more exactly, in pyrrole,MO, iron(III) chloride and water systems). Regardless of theirconclusions, there was a methodology of systematic investigation
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of the reactions presented, model sequences of the addition ofreactants, isolation and characterization of intermediates atvarious stages and the inuence of the temperature of synthesisand concentration of MO on the nal shape, electrical conduc-tivity and specic surface area of the nanotubes. In this contri-bution, we apply such a methodology (extended by measurementof the CMC) in order to investigate the inuence of three azo dyes:Acid Red 1 (AR), Orange G (OG) and Sunset Yellow FCF (SY),depicted in Fig. 1, on the morphology of the resulting supramo-lecular structures based on polypyrrole (abbreviated as PPy-AR,PPy-OG and PPy-SY). This selection of azo dyes was chosen dueto the similarity of their molecules. They contain a commonnaphthylphenyldiazene (marked in red in Fig. 1) moiety witha different conguration of polar substituents.
2. Experimental2.1. Synthesis of PPy structures
Pyrrole, iron(II) chloride, Acid Red 1, Orange G, Sunset YellowFCF, methyl orange (Sigma-Aldrich), and iron(III) chloridehexahydrate and hydrochloric acid (Penta) were used asreceived without any modications. The chemical structures ofthe azo dyes with a naphthylphenyldiazene skeleton used aredisplayed in Fig. 1.
The concentrations of the dyes used in the reactions fol-lowed the ascending sequence: 0.5, 2.5, 5, 10, 20, and 50 mM.An equimolar mole ratio of monomer to oxidant was kept. Ina typical synthesis, 10 mmol (1.63 g) of iron(III) chloride hexa-hydrate was dissolved in 200 ml of a 5 mM solution of azo dye indeionized water. The solution was kept at 5 �C using a thermo-stat, and then 700 ml of pyrrole monomer was added dropwise inthe rst two hours of synthesis. Thus, the molar concentrationsof the reactants were 50 mM for the iron(III) chloride hexahy-drate and 50 mM for the pyrrole.
The course of synthesis and material purication wasuniversal. During the synthesis (24 h), the mixture was stirred ata constant speed. The samples were puried by Soxhlet extrac-tion using acetone for several days until the extraction reagentremained colourless. Then the obtained powder was dried at40 �C in a vacuum dryer.
For comparison, the globular form of PPy (PPy-G) was alsoprepared without the presence of any azo dye from pyrrole(1 mM) and iron(III) chloride (1 mM) oxidant in an aqueousenvironment only. Iron(III) chloride was added dropwise intothe aqueous pyrrole solution at 5 �C for 24 h with gentle stirring.Also for comparison, standard nanotubular PPy was prepared inthe presence of MO as described earlier.21
The PPy synthesised using the recipes described above was(according to previous experiments25) considered as a poly-cation with the chloride and azo dye anions acting as thedopants. The former anion results from the oxidant – iron(III)chloride hexahydrate – and the latter result from azo dyes.
2.2. Preparation of binary systems
In order to systematically investigate the role of structure-guiding agents in the process of micro-structure synthesis, it
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Fig. 1 Azo dyes used as soft templates for the preparation of different PPy microstructures: (a) Acid Red 1, (b) Orange G, (c) Sunset Yellow FCFand (d) methyl orange for comparison.
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was necessary to simulate their formation during the variousstages of the reaction. So, the interaction of all three of the azodyes (AR, OG and SY) in binary systems with four chemicalsappearing in some stage of pyrrole polymerization – iron(III)chloride hexahydrate (oxidant), iron(II) chloride (the reducedform of the oxidant produced during the polymerization),hydrochloric acid (it is known that polymerization is connectedwith the production of protons and hence with the increasingacidity of the mixture) and pyrrole (monomer) – was studied.The samples were prepared by mixing the azo dyes and therelevant chemical at a mole ratio close to the situation in thepolymerization reaction.
2.3. Sample characterization
Optical microscopy (Nikon Eclipse LV100D with a digitalcamera, SONY DFW-SX910) in tandem with image analysis(soware NIS-Elements AR 3.20) was used to observe thestructure-guiding agents in the binary systems. Althoughoptical microscopy is limited by the dimensions of observableobjects, it proved to be a useful tool for in situ observation of thesupramolecular structures in the systems containing MO.21
Moreover, the dyes in aqueous solution were studied in terms ofthe surface tension by the Du Nouy ring method (TD1 LAUDA).Measurements were carried out at 7 �C, which is the tempera-ture approximately corresponding to the temperature used inthe pyrrole polymerization.
The morphology of the prepared microstructures wasobserved by scanning electron microscopy (SEM) using a MIRA3 LMH (Tescan Company) microscope with Schottky-typecathode at an accelerating voltage of 3 kV. Elemental analysiswas performed by energy dispersive X-ray microscopy (EDX)
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using a Quantax 200 analyzer with a XFlash 6|10 detector(Bruker Company) with a resolution of 129 eV and a 15 kVaccelerating voltage, respectively.
The Fourier-transform infrared (FTIR) spectra of powderedsamples dispersed in potassium bromide pellets were measuredusing a Thermo Nicolet NEXUS 870 FTIR Spectrometer witha DTGS TEC detector in the 400–4000 cm�1 wavenumber region.Raman spectra were recorded with a Renishaw InVia ReexRaman microspectrometer using a near-infrared diode 785 nmlaser excitation line. A research-grade Leica DM LM microscopewas used to focus the laser beam. The scattered light was ana-lysed with a spectrograph using a holographic grating with 1200lines mm�1. The Peltier-cooled CCD detector (576 � 384 pixels)registered the dispersed light.
The powders of synthesized PPy were compressed at 540MPato form pellets of 13 mm in diameter and 0.5–1 mm in thick-ness. Their electrical conductivity was then measured by thefour-point van der Pauw (VDP)method. The experimental set-upconsisted of a Keithley 220 programmable current source,a Keithley 2010 multimeter as a voltmeter, and a Keithley 705scanner equipped with a Keithley 7052 matrix card.
3. Results and discussion3.1. Overview of the synthesized samples
The resulting nano- and microstructures are displayed in theSEM images (Fig. 2–5). In principle, these forms appear: (1)rectangular nanotubes or nanorods of PPy-AR with cross-section dimensions ranging from tens of nanometres to unitsof micrometres and up to hundreds of micrometres in length(Fig. 2), and some of them possess multichamber cavities; (2)
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Fig. 2 Rectangular nanotubes prepared in the presence of AR (two magnifications).
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several micrometre large plates (Fig. 3) of PPy-OG combinedwith a globular structure; (3) several micrometre large barrel-like formations of PPy-SY (Fig. 4), some of these samples aremixtures of barrel-like formations, nanorods and globularstructures; (4) nally, structures prepared for comparison –
globular structures of PPy-G (Fig. 5a) and circular nanotubes ofPPy-MO (Fig. 5b). The simplied sketches in Fig. 6 show all ofstructures present in the synthesized samples. The images ob-tained by SEM therefore reveal an interesting inuence of theazo dyes used on the nal geometry of the nano- and micro-structures; the difference is particularly evident whencomparing the structures shown in Fig. 2–4 with the structuresshown in Fig. 5, where PPy-G and well-known PPy-MO arepresented.
Rectangular-shaped structures of PPy-AR were alreadyobserved in the past by Yan et al.13 (although at differentsynthesis conditions and at very high AR concentrations), butthe plates of PPy-OG or barrel-like structures of PPy-SY arefound, to the best of our knowledge, for the rst time. Asstructures other than common nanotubes such as wires or rods
Fig. 3 3-D structures prepared in the presence of OG.
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are rather uncommon in the case of PPy, the inuence ofnaphthylphenyldiazenes is worthy of further investigation anddiscussion.
Table 1 summarizes the basic properties (shape, electricalconductivity and approximate size) of all of the synthesizedsamples. Moreover, it also contains basic information about thestandard PPy-G and PPy-MO references synthesized eitherwithout azo dye or using MO, respectively.21 At rst glance, thevalues of conductivity of PPy-AR, PPy-SY and PPy-OG aremoderate (in the order of units of S cm�1) and comparable tothe conductivity of PPy-G (5.2 S cm�1) rather than to the value ofconductivity of PPy-MO (48.7 S cm�1).
The descriptions of the morphologies (Fig. 2–4) could bebased on comparisons of similar microstructures of otherconducting polymers. A plethora of papers are devoted tonanotubular polyaniline (PANI) or polythiophene (PEDOT), butonly a few papers present their 3-D microstructures.34 Some ofthem are oriented toward the supramolecular structures ofoligoanilines. Wang et al. published two papers devoted to oli-goaniline microstructures.35,36 He presumed that oligoanilines
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Fig. 4 3-D structures prepared in the presence of SY (two magnifications).
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are formed into highly organized structures which subsequentlyserve as seeds for the growth of polyaniline 3-D structures.Using this approach, Zhao et al.37 found highly ordered poly-aniline microstructures of extraordinary beauty. On the otherhand, 3-D structures of PPy are mostly known as a result ofhard-template synthesis, but only a few of them result from so-template synthesis, e.g. Santino et al.38 prepared PPy 3-Dnanobrushes by low-temperature vapour-phase synthesis.
Thus, the typical morphology of PPy synthesized in thepresence of a structure-guiding agent (e.g. surfactants or azodyes) is mostly of 1-D shape,39 hence the development ofstructures other than 1-D structures is surprising. According toone of the generally accepted theories, the nal shape of the PPysupramolecular structure is determined by the initial shape ofthe so-template which is developed from the structure-guidingagent in aqueous solution. Moreover, it seems that developmentof the so-template is connected with the transition of the azodye salt into an azo dye acid as reported in recent work.24
Therefore, in order to reveal the possible mechanism behindthis 3-D growth, we rst have to study the behaviour of naph-thylphenyldiazenes alone in aqueous solution.
Fig. 5 (a) PPy-G and (b) PPy-MO prepared in the presence of MO for c
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3.2. Study of dye solutions and template formation
The investigated azo dyes (Fig. 1; SY, AR and OG) are composedof a similar hydrophobic naphthylphenyldiazene skeleton andthey differ in the type and position of functional groups. Theypossess at least two hydrophilic sulfonic groups attached atdifferent positions on the naphthylphenyldiazene skeleton. Theexistence of distinct hydrophobic and hydrophilic partstogether with the possibility of hydrogen bond creation deter-mines the shape of their self-assemblies in aqueous media,which are stacked together due to both p–p and hydrophobicinteractions.37 Ionic bonds between the anion of the sulfonicgroup and the PPy polycation may also play some role duringthe polymerization of pyrrole.
Unfortunately, there is a lack of specic information aboutthe behaviour of the azo dyes AR, OG and SY in aqueous solu-tion. Taking into account the variability of particular concen-tration ranges and the degree of freedom of how dye moleculescan be mutually oriented,37 the shape of their self-assemblies isalmost unpredictable. Some insight into the issue of self-assembly of naphthylphenyldiazene has been given by
omparison.
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Fig. 6 The supramolecular structures that appear in the samples: (a) plates, (b) barrel-like formations, and (c) rectangular nanotubes; theirdimensions (d – diameter and l – length) are summarized in Table 1.
Table 1 Overview of synthesized samples
Concentration ofazo dye (mM) Morphology
Conductivity(S cm�1) Dimensionsa (nm)
No dyen/a Globular 5.2 n/a
Methyl Orange (reference)2.5 1-D 48.7 190–310b
Acid Red 10.5 Globular — n/a2.5 1-D 2.3 630 � 1805 1-D 2.7 630 � 22010 1-D 1.5 580 � 16025 1-D 0.2 820 � 270
Orange G0.5 Globular 8.4 n/a2.5 Globular 9.7 n/a5 3-D 10.9 8700–26 000
Sunset Yellow FCF0.5 Globular — n/a2.5 Globular — n/a5 3-D 3 6900–11 60010 1-D/3-D 0.8 270–860/8800–10 90020 3-D 12.7 n/a25 3-D 2.6 n/a50 3-D 8.3 n/a
a Diameter d in 1-D, size in 3-D. b From ref. 21.
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Horowitz et al.40 They investigated the behaviour of SY inaqueous solutions at different concentrations and found that SYbehaves as chromonic liquid crystals whose molecules arestacked by p–p interactions and they undergo a phase transi-tion to a separate ordered phase at high concentrations.
We have employed two tools in order to study aqueoussolutions of the naphthylphenyldiazene dyes: (1) opticalmicroscopy, which gives sufficient information about the so-template structure at high azo dye concentrations; (2)measurement of the surface tension which covers a wholeconcentration range and reveals important transitions in azodye solutions.
3.2.1. Optical microscopy of binary systems. A systematicstudy was carried out on template formation in the followingbinary systems: azo dye–hydrochloric acid, azo dye–iron(III)chloride and azo dye–iron(II) chloride.
Azo dye–hydrochloric acid. Interaction of azo dye with hydro-chloric acid simulates increasing the acidity of the native liquorduring the polymerization process. According to the litera-ture,21,31 increasing the acidity of the reaction mixture preventshydration of b-azo-nitrogen and, consequently, the azo dyemolecules escape from the bulk of the solution in the form ofsupramolecular structures. Moreover, the transition of methylorange salt into methyl orange acid in an acidic environment isreported in recent work24 as one of the reasons for templateformation. None of these phenomena were observed in the caseof SY, AR or OG. This means, that the acidity of the reactionsolution plays only a minor role in the synthesis of PPystructures.
Azo dye–iron(III) chloride. In the investigated systems, oblongtemplate structures were found in the AR–iron(III) chloridesystem (Fig. 7a); these structures (40–60 mm long and 1–2 mmwide) appeared only for high concentrations of AR and onlyaer longer periods of time (24 h). Similar structures werecreated in the OG–iron(III) chloride system (Fig. 7d). Theremaining system, i.e. SY–iron(III) chloride, did not form anytemplates that were observable by optical microscopy.
The molecules of azo dyes can be inuenced by iron(III)chloride in two ways: rstly, Fe3+ ions easily create complexespreferably with oxygen-containing ligand centres in azo dyemolecules and, secondly, addition of iron(III) chloride to thesolution will cause an increase in its acidity.13 As shown above,the acidity of the system plays only a minor role.
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Azo dye–iron(II) chloride. Iron(II) chloride is generated asa product of the reduction of iron(III) chloride during the poly-merization process. Of the studied systems, AR–iron(II) chlorideyielded large objects up to 600 mm long and 1–10 mm wide.
In the azo dye–iron(II) chloride system the situation issomewhat different than that in the previous case: Fe2+ ionscreate complexes preferably with nitrogen-containing ligandcentres of azo dye molecules and the acidity of the Fe2+ cation inthe water environment is lower compared with that of the Fe3+
cation.3.2.2. Surface tension. Measurement of the surface tension
by the Du Nouy ring method is a useful tool to study azo dyebehaviour in a particular concentration range (Fig. 8). Abrupt
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changes in surface tension indicate changes in molecularordering e.g. the creation of micelles due to hydrophobicattraction in aqueous solutions. All of the studied azo dyesdisplayed these changes at rather low concentrations, whichcorresponds to the fact that no structures were observed in thepolymerization reactions with low dye content. In our poly-merization reaction, the exact concentration point where so-template structures or seeds are created could be slightly shif-ted or fuzzy due to the contribution of pyrrole monomer.Moreover, it is known that SY and MO rst create p–p stackeddimers and oligomers at low concentrations32 and SY inparticular undergoes transition to columnar liquid crystals athigher concentrations. It is worth noting that supramolecularstructures of PPy-SY are created at concentrations around thistransition point, but not in low or very high concentrations ofSY. Therefore, we can speculate that the barrel-like shape ofPPy-SY was created by seeds of several stacked molecules of SYand not by the columnar phase of the SY liquid crystals.
3.3. Elemental analysis using EDX
Elemental composition is always depicted as a ratio ofelement’s atoms to the number of pyrrole rings. In order tocalculate the number of pyrrole rings, nitrogen originating fromthe azo dye has to be subtracted from the total nitrogen contentaccording to a procedure introduced by Yang et al.12
Determination of the elemental composition has revealedthat azo dye AR remains in the bulk of PPy-AR (similar to MO inPPy-MO) despite the intensity of extraction,25 i.e. the amount ofremnant AR molecules in PPy samples increases together withan increasing concentration of azo dye in the polymerization
Fig. 7 Binary systems of AR with: (a) iron(III) chloride, (b) iron(II) chloride,chloride.
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solution (up to an equimolar AR : pyrrole unit ratio). Thismeans that the composite of PPy-AR is created at high ARconcentrations, which usually has a negative inuence oncharge transport in supramolecular PPy, e.g. it decreased theconductivity of the samples (Table 1). EDX indicated the pres-ence of such a composite by a high sulfur content in thesamples (Fig. 9), as the only source of sulfur is the AR mole-cules.25 It is reasonable to assume that most of the remainingAR molecules are held in cavities of the nanotubes.13 On theother hand, PPy synthesized in the presence of SY containeda relatively steady concentration of sulfur in all of the samplesregardless of its initial concentration in the polymerizationmixture, which means that only a limited amount of SY mole-cules were held by PPy-SY. One of the reasons could be a lack ofsuitable cavities which are able to hold remnants of azo dye inthis structure (see Fig. 4).
The content of chlorine (Fig. 10) is the second importantparameter of supramolecular PPy which can be obtained byEDX measurement. Chlorine is usually incorporated in thematerial as chloride ions acting as counter-ions bonded tobackbone polycations, or it can be a sign of iron(III) chloride oriron(II) chloride remnants. Chloride ions usually indicate thedegree of protonation/deprotonation.1 In the case of the PPysamples synthesized in the presence of both AR and SY, thecontent of chlorine quickly reached zero with an increasing azodye concentration. Hence, for AR and SY the chlorine counteranions in PPy were replaced with anions of the respective azo-dyes. Moreover, there is no reason to believe that the chlorinewas part of the remaining iron(III) chloride or iron(II) chloridemolecules although a small amount of iron was found in theEDX spectra (up to 2 at%) of PPy-AR.
and (c) iron(II) chloride (higher concentration); and OG with (d) iron(III)
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Fig. 8 Measurement of the surface tension of the azo dyes: SY – change at 9.75 mM (downer left), OG – change around 0.2 mM (upper right),AR – change at 1.10 mM (upper left), and MO – featureless change (downer right).
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3.4. Measurement of conductivity
The conductivity of novel PPy structures is a crucial parameter fortheir future application. Despite the highly-ordered shapes ofPPy-AR, PPy-OG and PPy-SY, the electrical conductivity of thesepolymers is moderate, in the order of units of S cm�1 (Table 1).Such a value is comparable to that of PPy-G. We can discusspossible reasons why our samples do not exhibit conductivitysimilar to that of PPy-MO or higher. The rst one could be thepresence of low conducting residues, e.g. PPy-OG contains a highamount of unstructured globular PPy, and PPy-AR suffers from
Fig. 9 Content of sulfur in PPy-AR, PPy-OG and PPy-SY expressed asa ratio of the amount of sulfur to nitrogen in the pyrrole rings. Insetcontains the same ratio for PPy-MO;25 error bars were removed forbetter orientation in the graph.
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a higher content of AR remnants. These would reduce the overallconductivity by mixing with the (hypothetically) well-conductingstructured PPy. Another reason, as shown by Long et al.,2 couldbe high contact resistance between individual supramolecularstructures which can superimpose the intrinsic low resistance ofhighly ordered polymer chains of structures. When we considerthe sizes and complex shapes of PPy-SY, PPy-OG and PPy-AR incomparison with those of PPy-MO, we should presume thissecond reason to be the more important one.
Besides the value of conductivity, temporal stability isanother important parameter for their use. Fig. 11 depicts the
Fig. 10 Content of chlorine in PPy-AR, PPy-OG and PPy-SY expressedas a ratio of the amount of chlorine to nitrogen in the pyrrole rings.Inset contains the same ratio for PPy-MO.
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time dependence of the conductivity of the polypyrrole samples.It is apparent that, in principle, all of the samples follow thesame generalized “aging curve”, regardless of the type of azo dye(or even its presence). When comparing the half-life time t0.5(dened as the aging time required for the conductivity to behalved from the initial value s0, s(t0.5) ¼ 0.5s0) of all of thesamples (excluding PPy-OG because of the high PPy-G content),it is clear that PPy synthesized in naphthylphenyldiazenespossesses stability comparable to that of PPy-G (t0.5 is approx.70–80 days). Thus, no stabilizing effect of the azo dye wasobserved unlike in the case of PPy prepared in the presence ofmethyl orange (t0.5 is approx. 400–450 days) where the stabilitywas greatly enhanced with an increasing MO concentration.41
Both its moderate conductivity and its stability probably reectthe highly-disordered state of PPy in all of the samples typicalfor PPy-G and contrary to PPy-MO which contains well-orderedhighly conducting regions.42
3.5. FTIR and Raman spectroscopy
The infrared spectrum of standard PPy-G prepared without anydye (Fig. 12) exhibits the previously described main bands.43–45
The weak band at 1700 cm�1 corresponds to the presence ofa carbonyl group attributed to the nucleophilic attack of waterduring the preparation.45 We detect the maximum situated at1536 cm�1 (C–C stretching vibrations in the pyrrole ring), thebroad band with maxima at 1472 and 1445 cm�1 (C–N stretch-ing vibration in the pyrrole rings), and the maximum at1295 cm�1 (C–H or C–N in-plane deformation modes), and thebroad band with the maximum situated at 1165 cm�1 (thebreathing vibrations of the pyrrole rings). We observe the peakat 1092 cm�1 (N–H+ deformation) and a sharp band at1037 cm�1 (C–H and N–H in-plane deformation vibrations).Bands situated at 964 cm�1 (C–C out-of-plane ring-deformation), at 895 cm�1 (C–H out-of-plane deformation ofthe ring) and at 782 cm�1 (C–H out-of-plane ring deformation)are also present in the spectrum.
Fig. 11 Measurement of the time dependence of the conductivity ofthe PPy samples. The legend displays the initial values of conductivity.
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The infrared spectra of nanostructured PPy prepared in thepresence of azo dyes (Fig. 12) only slightly differ from thespectrum of globular PPy. We detect some small peaks of dyesin the spectra. Very small peaks of OG, few higher peaks and theshi of the peaks at 1536 and 1295 and 1037 cm�1 to higherwavenumbers in case of SY and AR are observed. For PPyprepared in the presence of methyl orange we observe practi-cally the spectrum of the dye, which corresponds to the acidicform of methyl orange.21,23,24 The shis of the main bands of PPycorrespond most probably to overlapping with the peaks of thecorresponding dyes. The shape of the spectra, especially in theregion above 1600 cm�1, corresponds to the decreasingconductivity in the order PPy-G > PPy-OG > PPy-SY > PPy-AR,1
except for PPy-MO, which has the highest conductivity andhighest peaks of MO in the spectrum. Raman scattering isa more convenient spectroscopic method to study the presenceof polarons and bipolarons in the PPy structure.
The Raman spectrum of standard PPy-G prepared withoutany dye (Fig. 13) exhibits the previously described mainbands.1,23,24,46–48 We observe the band at 1596 cm�1 (C]Cstretching vibrations of the PPy backbone), two bands of ring-stretching vibrations at 1381 and 1326 cm�1 (the intensity ofthe latter increases aer deprotonation), the band at 1240 cm�1
(antisymmetric C–H deformation vibrations) and the doublepeak with local maxima at 1082 and 1045 cm�1 (C–H out-of-plane deformation vibrations, the second one becomessharper in the case of deprotonation) in the spectrum. The bandat 972 cm�1 (the ring-deformation vibrations of neutral poly-pyrrole units) and the sharp peak at 934 cm�1 (the ring-deformation vibrations in dication–bipolaron units) are well-detected in the spectrum.1,43,46–48
In the Raman spectra of PPy prepared in the presence of azodyes (Fig. 13) we observe the same bands practically at the samepositions as in the spectrum of PPy-G. This means that bothforms have identical molecular structure and may differ only inthe organization of polymer chains. The energy of the laser
Fig. 12 FTIR spectra of polypyrrole prepared without any dye (PPy-G)and in the presence of 5 mMOG, 5 mM SY, 5 mM AR, and 3.5 mM MO.The spectra of corresponding dyes are shown for comparison.
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Fig. 13 Raman spectra of polypyrrole prepared without any dye (PPy-G) and in the presence of 5 mM OG, 5 mM SY, 5 mM AR, 3.5 mM MO.The spectra of corresponding dyes are shown for comparison. Thelaser excitation wavelength is 785 nm.
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excitation line 785 nm is in resonance with the energy of thedelocalized polarons and bipolarons in PPy and the spectrumobtained with the laser excitation at 785 nm corresponds to thesurface of the structured PPy. We can conclude that dyes arepresent mostly inside of them. From the ratio of the intensitiesof the peaks at 1082 and 1045 cm�1 we can conclude that thedoping level of the surface of the samples decreases from PPy-OG > PPy-SY > PPy-AR > PPy-MO > PPy-G. As the highestconductivity exhibit the samples of nanotubular PPy-MO andPPy-G, we suppose that the charge transport proceeds prefer-ably inside the structures where PPy is in contact with dyes.
4. Conclusions
The azo dyes Acid Red 1, Orange G and Sunset Yellow FCF, withnaphthylphenyldiazene skeletons, were used as so-templatesfor the preparation of PPy in aqueous solution. The resulting“conventional” 1-D and “novel” 3-D supramolecular structureswere observed by SEM and their properties (chemical compo-sition, structure and electrical conductivity) were compared tothose of standard globular PPy and also to those of the well-known nanotubular PPy prepared in the presence of methylorange. A deeper insight was also made into the nature of so-template formation.
Based on measurements of the dependence of surfacetension on azo dye concentration, it can be concluded thatsupramolecular structures of neat morphology are formed onlynear the point of abrupt change of surface tension (which isusually referred to as the “critical micelle concentration”).Neither lower nor higher concentrations lead to neatermorphology with enhanced properties. Lower concentrations ofazo dyes result in a mixture of globular PPy and its supramo-lecular structures, and higher concentrations usually lead to theformation of a composite of azo dye and PPy.
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The shape of the PPy supramolecular structures can becontrolled using azo dyes with different chemical structuresand, besides the nanotubes, novel PPy supramolecular struc-tures can be expected when using structure-guiding agents froma large group of the azo dyes that have not been tested so far.However, it is still a challenge to predict the nal shape of thePPy supramolecular structure when the positions of the func-tional hydrophilic and hydrophobic groups of the azo dye areslightly changed compared to those azo dyes whose structure-guiding behaviour is well known. Easier prediction is expectedin the case of azo dyes creating solid crystals in aqueous solu-tions which can be observed by e.g. optical microscopy, but suchprediction is very difficult in the case of the azo dyes creatingliquid crystals.
Unfortunately, none of the as-presented supramolecularstructures of PPy possess better conductivity and long-termstability than nanotubes of PPy prepared in the presence ofmethyl orange.
However, this work illustrates a great potential which is stillhidden in the polymerization of PPy using structure-guidingagents from the group of azo dyes. This area is still far frombeing investigated (and much less understood or interpreted).We can conclude that at least the morphology of synthesizedPPy is an easily and reproducibly tunable parameter. Even in thesmall set of azo dyes presented in this paper (MO, AR, OG andSY), rectangular nanotubes, rectangular nanorods, at plates,barrel-like formations, circular nanotubes or globules weresynthesised. Such results can be considered as a contribution tothe bottom-up approach of modifying PPy morphology.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors thank the Czech Science Foundation (17-04109S,17-13427S) for nancial support. This research was also spon-sored by the NATO Public Diplomacy Division in the frameworkof “Science for Peace” (984597).
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